Simplify the following expression: $q = \dfrac{9}{8n} - \dfrac{1}{5n}$
In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $8n$ and $5n$ $\lcm(8n, 5n) = 40n$ $ q = \dfrac{5}{5} \cdot \dfrac{9}{8n} - \dfrac{8}{8} \cdot \dfrac{1}{5n} $ $q = \dfrac{45}{40n} - \dfrac{8}{40n}$ $q = \dfrac{45 -8}{40n}$ $q = \dfrac{37}{40n}$